Intercommunication system



March 1, 1966 s. CURCHACK 3,233,302

INTERCOMMUNICATION SYSTEM Filed July 5, 1962 3 Sheets-Sheet 1 19 INVENTOR. SAMUEL Cage/ 4 cK BY V AZOIQNEYS March 1, 1966 S. CURCHACK INIERCOMMUNICATION SYSTEM Filed July 5, 1962 Tiq. 2

figs r 5714 no;

3 Sheets-Sheet 2 f l 15 I AMPLIFIER I 9 7 '28:.)- l 24 R3 I 28 1 I Q 1 l l l J INVENTOR.

SAMUEL CUQCH/ CJ March 1, 1966 s, cuRc g 3,238,302

INTERGOMMUNIGATION SYSTEM Filed July 5, 1962 3 Sheets-Sheet 3 Fqom SrA 770/) Z N/ W INVENTOR.

SAMUEL C Ugo-M cl 2 A ENEYS United States Patent 3,238,302 INTERCOMMUNICATION SYSTEM Samuel Curchack, Whitestone, N.Y., assignor to Fanon Electronic Industries, Inc., Newark, NJ., a corporation of New York Filed July 5, 1962, Ser. No. 207,697 8 Claims. (Cl. 1791) This invention relates in general to an electronic intercommunication system and more particularly to a switchless two-way communication system.

There are many two-way communication systems known to the art. A major problem in the art is the avoiding of acoustic feed back which results in howl. The howl results from the 'fact that the microphone at each station is located close to the speaker at that station. Thus, in the absence of some technique for disabling one of the speakers or microphones, the sound from a speaker is picked up by the nearby microphone and is transmitted to the speaker at the other station and through the microphone at the other station backto the first station. This acoustic feed back results in a howl which makes such a simple intercommunication system completely useless.

The usual prior art solution to the acoustic feed back problem was to incorporate some sort of a switch, generally manually operated, to disable or enable one or the other of the microphone-speaker combinations. For example, a switch at each microphone would have to be turned on by the user when he wished to speak into the microphone.

Manually actuated switches have the disadvantage of requiring that the user remember to use a switch. All types of switches have the disadvantage of making it impossible for the party at one of the stations to cut in on the party speaking at the other station. Thus, the previously known communication systems impose a barrier to normal conversational give and take.

Accordingly, it is a major object of this invention to provide an intercommunication system which will permit normal cross-conversation while avoiding acoustic feed back.

It is a related object of this invention to provide a switchless intercommunication system.

It is another object of this invention to provide such a switchless intercommunication system without requiring extensive and expensive additional circuitry. The convenience ofhaving normal cross-conversation possible in an intercommunication system does not normally warrant greatly increased expenditures in such systems and thus a practical system must be one that can be manufactured at only slightly greater cost than the prior art systems which incorporate switches.

In brief, the device of this invention involves rectifying a portion of the output of each amplifier to provide a bias, which bias .is then used to decrease the gain of the other amplifier. Thus when one party is speaking, the output of his amplifier is used to bias the amplifier connected to the microphone at the receiving station. Ac-

7 cordingly, the sending partys voice as it comes from the 3,238,302 Patented Mar. 1, 1966 The reverse bias rectification circuit has a slight time delay so that normal speech hesitation does not remove the bias. Since the bias arrangement merely decreases the gain of the amplifier associated with the receiving station, the listener at the receiving station can interrupt the sending party by speaking into his microphone with a voice that is loud enough to provide an appreciable output from his amplifier and thus provide a reverse bias at the sending partys amplifier. The receiving party can in this fashion interrupt the sending party and reverse roles. However, as soon as this receiving party has established the domination of his circuit, he can speak in a normal conversational tone which permits the first sending party to re-interrupt in the same fashion as did the first receiving party.

Other objects and purposes will become apparent from the following drawings and detailed description, in which:

FIG. 1 is a block and schematic diagram of this invention;

FIG. 2 is a .block and schematic diagram showing an improved version of the invention;

FIG. 3 is a schematic diagram illustrating the circuit arrangements at one station;

FIG. 4 is a simplified mechanical illustration of the preferred mounting for the diaphragms incorporated in the speakers used in FIG. 1; and

FIG. 5 is a simplifier mechanical illustration of a preferred arrangement for the microphone used in FIG. 1.

FIG. 1 illustrates an embodiment of the invention, illustrating a two station intercommunication system. The first station 10 includes a microphone 11, the output of which .is fed into and amplified by an amplifier 12. The output of the amplifier 12 is fed to a speaker 13 at the second station 14. Thus a person at the first station 10 communicates to a person at the second station 14 through the microphone 11, the amplifier 12 and the speaker 13. Similarly, a person at the second station 14 communicates to a person at the first station 10 through the microphone 15, the amplifier 16 and the speaker 17.

To avoid howl and acoustic feed back, it is necessary that the amplifier 16 be cut oif or have its gain materially reduced when someone is speaking into the microphone 11; and similarly, it is necessary that the amplifier 12 be cut off or have its gain materially reduced when someone .is speaking at the microphone 15. This invention achieves the appropriate amplifier gain reduction by a technique which avoids the use of manual switches and which operates entirely in response to the voice level at microphones 11 and 15.

In simplest terms, this amplifier gain reduction is achieved by using a portion of the output of each amplifier 12, 16 as a signal to be fed to the other amplifier 16, 12 in such a fashion as to reduce the gain of the other amplifier 16, 12. Thus when a party at the first station 10 speaks into the microphone 11, the amplifier output 12 is used in part to provide the acoustical output at the speaker 13 and in part to provide an input to the amplifier 16 which input reduces the gain of the amplifier 16. In this situation, the signal from amplifier 12 reduces the gain of amplifier 16 well before the acoustical output of the speaker 13 reaches the microphone 15 and thus howl and acoustic feed back is avoided. Speaker 13 and microphone 15 are normally so arranged that attenuation of the acoustic signal fed to microphone 15 from speaker 13 is as great as possible. This attenuation, combined with the reduction in gain of amplifier 16 by the signal from amplifier 12 renders the loop gain of the system sufficiently low so as to avoid acoustic oscillation or howl. The speaker 13 and microphone 15 must be placed apart sufiiciently far so that there will be an acoustic lag between the speaker 13 and the microphone 15 greater than the time necessary to decrease the gain of the amplifier 16.

A comparable sequence of events occurs when someone speaks into microphone 15.

The signal from the amplifier 12 serves to decrease the gain of the amplifier 16 by applying a reverse bias to the first stage of the amplification circuits in the amplifier 16. The reverse bias is achieved through use of a rectifier 2t) and RC filter R C R Since both stations and 14 operate in the same fashion, the same numerals will be used to designate similar circuit parts. In addition, the RC circuit R C R is used to achieve a slight delay in imposing the reverse bias and a considerably longer delay in removing the reverse bias on the amplifiers 12, 16. The delay in imposing and removing the reverse bias assures a smooth change in gain so as to avoid undesirable popping noises during the course of one persons talk and during the switch over from the party at one microphone 11, 15 to the party at the other microphone 15, 11.

In connection with this problem, it must be remembered that the application of reverse bias to lower the gain of the amplifier is not a cut-off operation in the preferred embodiment. An advantage of a system which does not employ complete cut-off of the amplifier not immediately in use is the minimization of the annoying noises that occur as the amplifiers alternately cut in and cut out.

If we assume that an operator is speaking into the microphone 15 so that an output is provided by the amplifier 16, there will be a signal which will cause the speaker 17 to provide the desired acoustic signal. In addition, the signal output from the amplifier 16 will partially be diverted through the circuit consisting of a rectifier and an R C R filter network to provide the desired rectified reverse bias for turning down the gain in the amplifier 12.

The diode 20 is necessary to convert the alternating amplifier 16 output into a signal having a rectified or DC. component. The R C circuit operates to create a very slight delay in the application of the bias to the amplifier 12 so that the amplifier 12 will not present a popping noise as it is turned down. However, the R C time constant must be considerably less than the time it takes an acoustical signal to go from the speaker 17 to the microphone 11 so as to assure that the amplifier 12 has been turned down in time to avoid acoustic feed back.

In reverse, this double time delay circuit R R C has a considerably longer time delay. The R C time delay circuit is preferably designed for a 0.4 second time delay. The 0.4 second R C circuit assures that the reverse bias is not removed from the amplifier 12 because of normal speech hesitation. In effect, the rate at which the amplifier 12 regains its full gain is at a rate slower than normal speech hesitation and thus the annoying pops that would otherwise appear are avoided. The diode 20 serves a function in connection with this R C time delay circuit in that the diode 20 holds off the discharge of C during short periods of no signal so that the time delay constant is determined by R C rather than R C discharging through the speaker 17.

This dual function for the diode 20, its operation as a rectifier to bias the amplifier 12 coupled with its operation as a. nearly infinite reverse resistance to assure that the bias removal will be by discharge of C through R makes this circuit uniquely simple, reliable and economical.

As contrasted with the control amplifiers and other complex circuitry that might be used in switchless intercommunication systems, FIG. 1 illustrates the extremely simple, reliable and inexpensive circuit additions necessary to be called for by this invention. The output of each amplifier 12, 16 is fed to its respective speaker 13, 17 and a portion of each output is concurrently fed as a reverse bias to a stage in the other amplifier 16, 12 respectively. The output of one amplifier 12, 16 is rectified by a diode 20 and then fed through a filter and time delay RC network R C R One major advantage of the above arrangement for tuning down the listeners amplifier is that the circuitry can be incorporated into use with presently used amplifiers and speakers. Thus this invention can be a modification to installations now in existence or can be incorporated into newly manufactured installations.

It facilitates conversation between the parties using the two microphones 11, 15 if the amplifiers 12, 16 have substantially the same gain. With similar gain amplifiers 12, 16 it becomes fairly easy for one party to interrupt the other as in normal conversation to ask questions, clarify points and engage in the usual give and take of conversation. If we assume that the person at microphone 11 is speaking and thus that the amplifier 12 is at full gain while the amplifier 16 is biased to a fairly low gain, it becomes possible for the person at the second station to interrupt the first party at any time by speaking into the microphone 15 at a somewhat louder than normal voice thereby providing sufficient output from the amplifier 16 to bias the amplifier 12 to a low gain, and attract the attention through the speaker 17 of the person at station one. Concurrently, the initially increased output of the amplifier 16, by biasing the amplifier 12 to a low gain, decreases the output of the amplifier 12 sufiiciently so that the bias from the amplifier 16 is substantially removed. Within the period of the R C time delay (approximately 0.4 second) the party at the second station has a dominant acoustical position and can continue to speak in a normal tone of voice until or unless the party at the first station re-interrupts by initially speaking in a voice appreciably louder than the voice being used by the party at the second station.

In this connection, it should be noted that if the design is such that the incoming signal to the amplifier not immediately in use is strong enough to completely bias the amplifier not in immediate use to an off condition, then it becomes impossible for the listening party to interrupt the speaking party. Thus within the inventive concept so far described there are two major variations which can be designed into the system, one is to provide such complete cut-off that it becomes impossible for one party to interrupt the other, and the second variation to provide only a gain reduction bias so that it is possible for one party to interrupt the other. The particular design chosen would depend upon the application. However, it is contemplated that the most useful application to this invention would involve situations where it would be preferable to have the persons at each station in a position to interrupt each other.

Under normal condition of operation, both amplifiers 12 and 16 are on at all times. It is just that one of these two amplifiers 12, 16 is biased to a low gain condition so that the speaker associated with that amplifier will not provide an appreciable acoustical output. Under these normal conditions, the individual who is at the microphone 11, 15 associated with the amplifier 12, 16 which is biased below gain can, by speaking relatively loudly into that microphone 11, 15, interrupt the other party at the other microphone 15, 11. The interrupting party causes two events to occur almost simultaneously, name- 1y: (1) he creates a bias across the amplifier 16, 12 of the other station thereby lowering the gain of that amplifier, and (2) because its output is lowered his own stations gain is re-established.

The above described procedure whereby the individual at one station can interrupt the individual speaking at the other station contrasts with the usual approach of prior art intercommunication systems. Prior art systems typically provide each party with his own exclusive control over his amplifier, such as by a switch on each microphone.

An intercommunication system as described, having the appropriate parts parameters, will operate somewhat differently under three different types of operating conditions, specifically:

(l) The usual operating condition under which the individual at one of the stations speaks loudly enough to lower the gain in the amplifier at the other station and then continues to speak in normal conversational tone until interrupted by the other individual at the other station who may break in by initially speaking loudly enough to reverse the amplifier gain conditions and who may then continue to speak in a normal conversational tone. Under these conditions, the individual who wishes to break into the discourse coming from the other station must initially speak more loudly than the individual at the other station. Of course, under this normal operating condition, if the individual who is speaking ceases to speak for an appreciable period of time (approximately 0.4 second in the preferred design) then the individual at the other station may start speaking at his regular conversational tone since the bias which had been keeping the gain of his amplifier below level will have bled on.

(2) If the individuals at bothstations speak in a sufficiently low voice, then it is possible to have both amplifiers sufficiently operative so that the two speakers 13, 17, will operate simultaneously. Under these conditions, the speaker 13, 17, output will generally be i'nsufii-cient to cause the undesirable acoustical feed-back.

(3) If the individual who is speaking from one of the stations talks loudly enough, then he may create a strongenough bias on the arnplifier at the other station so as to completely cut off that amplifier and thus make it impossible for the individual at the other station to cut in regardless of how loudly that individual may speak.

The above three operating conditions are described to give a comprehensive "picture of the operation of the device of this invention. However, the method of operation of the device of this invention is "most suitable for the first of the above three described operating conditions. Indeed, this invention has been made in order to make possible the so-called normal method of operation described above.

The above description indicates that the magnitude of the reverse bias applied to the listener station amplifier is a direct function of the volume of the sending station output and thus that the gain of the listener station amplifier is an inverse function of the sending station output volume. In the claims the term function will be used in its broadest mathematical sense to mean a relationship between two values whether that relationship be direct or inverse, linear or non-linear.

It is preferable, though notessentiaL that the amplifier 12, 16 not be volume controlled so that the amplifier will provide full volume for the purpose of developing a reverse bias. It is thus desirable to place the volume control on the speaker 13, 17. The volume control can then be manually operated by the listener to provide whatever desired volume he requires. If the volume control were incorporated'into the amplifiers 12, 16 then the full output of the amplifiers would "not be available at all times and the reverse bias applied to the unused amplifier would be decreased correspondingly. This decrease in reverse bias would mean a lessened reduction in amplifier gain and accordingly a less effective system, unless complex additional circuitry were added.

FIG. 2 illustrates a circuit which when added to the system of FIG. 1 results in a further improvement in the operation of this switchless intercommunication system. FIG. 2 illustrates a microphone 21, amplifier 22, and speaker 27 which perform the same function as the corresponding units in FIG. 1. The reverse biasing circuitry illustrated in FIG. 1, constituting a diode 20 and the delay circuit R C R is not shown in FIG. 2 in order to simplify the diagram and explanation. FIG. 3 illustrates both the circuitry of FIG. 1 and the circuitry of FIG. 2 incorporated with a complete circuit diagram for an amplifier. The following description will relate to the added circuitry at the first station, it being understood that the circuitry at the second station is identical and operated in the same fashion.

The arrangement in FIG. 2 takes advantage of the fact that acoustic oscillation (howl) usually occurs at frequencies above 1 kc. The circuit additions in FIG. 2 make the amplifier 22 relatively insensitive to these higher frequencies unless the amplifier 22 is producing an electrical output.

The capacitor C is selected to have a low reactance at frequencies above 500 c.p.s. Thus those frequencies that are emitted from the speaker 23 that would tend to cause oscillation are effectively shorted to ground through the capacitor C and the diode 28. Since it is important that the higher frequencies not be so shorted to ground when the microphone 21 is being used to transmit information through the amplifier 22 to the speaker 27, there is provided a means to cancel the bias on the diode 28 when the microphone 21 is being used. The cancelling of the bias on the diode 28 refers to applying a positive voltage to the cathode 28a so that the diode 28 no longer presents a ready path to ground.

It should be remembered that the gain on the amplifier 22 Will normally be low because of the reverse biasing effect described in connection with FIG. 1 when information is being transmitted from the second station to the first station. When the gain on the amplifier 22 is restored to normal either because the party at the second station has ceased to speak or because the party at the first station has asserted his dominance by initially speaking loudly into the microphone 21 (as described in connection with the circuit of FIG. 1) the amplifier 22 will provide an appreciable output in response to the first station party speaking into the microphone 21. A portion of the output of the amplifier 22 is diverted through the circuit consisting of a rectifier 29' and an R C R filter network to provide the desired positive DC. voltage on the cathode 28a of the diode 28. Thereby, the higher frequency shunt effect of the capacitor C is removed during use of the microphone 21.

The typical diode 28 will have an appreciable static forward internal resistance unless a small negative bias is applied to its cathode 2811. Accordingly, the cathode 28a is connected, through the current limiting resistor R to a negative potential within the amplifier 22. During non-use of the microphone 21, the negative potential on the diode 28 assures that the diode 28 is conducting. Since the diode 28 has a low dynamic resistance, the capacitor C is effectively at ground potential. The positive voltage developed by the signal rectified through diode 26 is used to cancel this negative voltage so as to regain the resistance across the diode 28 and thus avoid the shunting effect of the capacitor C while the microphone 21 is in use.

Because it is important that the shunting effect of the capacitor C be obviated as soon as the party at the first station starts to use the microphone 21, it is important that the time constant R C be short. However, it is also important that the resistance R be large enough to prevent loading of the amplifier 22 output. Thus a balance must be struck between the objective of a short time constant R 0 and the objective of minimum loading on the amplifier 22 output.

It is desirable that the time constant (R +R )C be fairly long (3 seconds is desirable) so that there will be no chance of losing the positive voltage applied to the cathode 28a while the party at the first station is speaking into the microphone 21. A short time constant (R +R )C might result in intermittent shortening of the higher frequency tones in the microphone 21 output.

It should be remembered that when the party at the second station is speaking into his microphone 25 and dominating the system, that the output of the amplifier 26 at the second station would be in part used to reverse bias the amplifier 22 and thus decrease the gain of amplifier 22. Under such conditions, the receipt at the microphone 21 of the acoustical output from the first station speaker 23 will not be sufficiently amplified by the amplifier 22 to provide an output adequate to develop a significant positive voltage at its diode 28 cathode 28a. Accordingly, the shorting circuit through C will be effective only when the amplifier 22 has no appreciable output and that depends upon the reverse biasing of the amplifier 22 by the diode 20, R C R circuit. Thus the circuitry described in connection with FIG. 2 must be considered an improvement on the basic circuitry described in connection with FIG. 1 since the proper operation of the FIG. 2 circuitry depends upon the operation of the FIG. 1 circuitry.

The complete circuit diagram illustrated in FIG. 3 relates the specific circuitry illustrated in FIGS. 1 and 2 to the circuitry of a typical transistor amplifier. It should be noted that a transistorized amplifier is a preferable type of amplifier for use in connection with this invention. The advantage of the transistor over the electron tube in this application is that the transistor requires only a relatively small bias change on its base to materially change its gain.

In the FIG. 3 amplifier PNP transistors are illustrated. If NPN transistors were used, the diode 20 connection would be reversed in order to develop a bias of the proper polarity to decrease transistor gain.

The critically of the circuit may be further reduced by the manner in which the speaker diaphragm and microphone are mounted.

For example, as illustrated in FIG. 4, the speaker diaphragm 31 may be mounted somewhat away from the baffle 29 by posts 30 so that any small amount of low frequency amplifier output, while the amplifier is reverse biased to low gain, will result in an acoustical output that tends to cancel itself. The acoustical cancellation occurs because the output from the face 34 of the diaphragm will in part be transmitted to the back 35 of the diaphragm 31 and thus cause acoustical cancellation. The shunting action of the capacitor C above described, takes care of the high frequencies.

If the microphone 11, 15 is mounted in a perforated box 34, as illustrated in FIG. 5, then the output of the speaker in operation will tend to impinge on both sides of the microphone diaphragm 35 and thus cause a measure of acoustical cancellation at higher as well as lower frequencies. However, when the party at the microphone speaks directly into that microphone, his voice will substantially impinge on the front surface 36 of the diaphragm 35 and thus result in a minimum of acoustical cancellation.

Although the circuitry and techniques illustrated in FIGS. 2, 4 and 5 are all an important part of the preferred embodiment of this invention, it must be realized that their operability, in the case of the cicuitry of FIG. 2, and their effectiveness in the case of the expedients illustrated in FIGS. 4 and 5, depend upon the design illustrated in FIG. 1.

The primary application for this invention is to the ordinary audio intercommunication system. However, there may be applications to a wireless intercommunication system, such as a walkie-talkie. Any such radio intercommunication system to incorporate this invention would require that separate transmitting and receiving circuits be used.

What is claimed is:

1. In an intercommunication system first station having a microphone, an amplifier coupled to said microphone to provide a station output and a speaker adapted to receive a station input from a second station, said first station also having circuit means for coupling a portion of the input of said speaker to said amplifier to reduce the gain of said amplifier as a function of the magnitude of the information signal supplied to said speaker, the improvement comprising:

bypass circuit means coupled to the output of said microphone to provide a bypass for a predetermined upper frequency range of the output of said microphone, and

disenabling circuit means coupled to the output of said amplifier to provide a signal to disenable said bypass circuit means when the output of said amplifier attains a pre-determined volume level;

2. The invention of claim 1 wherein said disenabling circuit means has a first time constant on the build-up of said signal in response to an increase in the output of said amplifier and a second time constant on the decay of said signal in response to a decrease in the output of said amplifier, said first time constant being less than said second time constant.

3. An intercommunication system first station having a microphone, an amplifier coupled to said microphone to provide a :station output and a speaker adapted to receive a station input from a second station, said first station also having circuit means for coupling a portion of the input of said speaker to said amplifier to reduce the gain of said amplifier as a function of the magnitude of the information signal supplied to said speaker, the improvement comprising:

a shunting circuit connected across the output of said microphone, said shunting circuit including a capacitor in series with a diode, one terminal of said capacitor being connected to the output of said microphone,

means for applying a first bias signal to the juncture between said capacitor and said diode, the polarity and magnitude of said first bias signal being sufficient to render said diode conducting so that said shunting circuit will serve as a bypass for a predetermined upper frequency range of the output signal of said microphone, and

disenabling circuit means coupled to the output of said amplifier to provide a second bias signal at the juncture of said capacitor and said diode, said second bias signal having a polarity opposite to said first bias signal and having a magnitude that is a function of the output volume of said amplifier, whereby said second bias signal will cancel said first bias signal when said amplifier attains a pre-determined volume level and thus cause said diode to become non-conducting so that said upper frequency range of microphone output signals will become an input to said amplifier when the output of said amplifier attains said pre-determined volume level.

4. The invention of claim 3 wherein said disenabling circuit means has a first time constant on the build-up of said second bias signal in response to an increase in the output of said amplifier and a second time constant on the decay of said second bias signal in response to a decrease in the output of said amplifier, said first time constant being shorter than said second time constant.

5. In a station adapted for use in an intercommunication system, said station having a microphone, an amplifier coupled to said microphone to provide an amplified output signal, a speaker adapted to receive an incoming signal, and means coupling a portion of the input of said speaker to the input stage of said amplifier to rectify a portion of whatever input signal is supplied to said speaker thereby providing a bias on said input stage of said amplifier to decrease the gain of said amplifier when an input is received by said speaker, the improvement comprising:

a shunting circuit connected across the output of said microphone, said shunting circuit including a capacitor in series with a diode, one terminal of said capacitor being connected to the output of said microphone,

means for applying a first bias signal to the juncture between said capacitor and said diode, said first bias signal having a polarity to render said diode conducting, and

circuit means connected at one end to the output of said amplifier and at the other end to said juncture between said diode and said capacitor to apply a second bias signal opposite in polarity to said first bias signal, the magnitude of said second bias signal being a function of the magnitude of the output of said amplifier, said circuit means having a first time constant on the build-up of said second bias signal and having a second time constant on the decay of said second bias signal, said first time constant being substantially shorter than said second time constant.

6. An intercommunication system comprising a first station having a first microphone and a first amplifier, the output of said first microphone being coupled to the input of said first amplifier,

a second station having a second microphone and a second amplifier, the output of said second microphone being coupled to the input of said second a-mplifier,

first adjusting means coupling a portion of the output of said second amplifier to said first amplifier for adjusting the gain of said first amplifier as a function of the magnitude of the information signal in said second amplifier output,

second adjusting means coupling a portion of the output of said first amplifier to said second amplifier for adjusting the gain of said second amplifier as a function of the magnitude of the information signal in said first amplifier output,

each of said adjusting means comprising a rectifying circuit to provide a bias that serves to decrease amplifier gain, said rectifying circuit including a diode having a first terminal connected to the output of one of said amplifiers, a first resistor connected at its first end to the second terminal of said diode, a second resistor connected at its first end to the second end of said first resistor and at its second end to the input stage of the other of said amplifiers, and a capacitor connected at its first end to said second end of said first resist-or and at its second end to ground,

a shunting circuit, one each connected across the output of each of said microphones, said shunting circuit comprising a capacitor in series with a diode, said diode being connected to ground and one terminal of said capacitor being connected to the output of said microphone,

means for applying a bias to the ungrounded terminal of said diode, and

circuit means connected at one end to the output of said amplifier and at the other end to said ungrounded terminal of said diode to apply a voltage tending to cancel said bias, the magnitude of said canceling voltage being a function of the magnitude of the output of said amplifier, said circuit means having a relatively short time constant on the build up of said canceling voltage from said amplifier output, and having a relatively long time constant on the discharge of said canceling voltage.

7. An intercommunication system comprising a first station having a first microphone, a first amplifier and a first speaker,

a second station having a second microphone, a second amplifier and a second speaker,

wherein the information signal output of said first microphone is coupled to the input of said first arnplifier to provide an amplified information signal output of said first amplifier, which amplified information signal is coupled to the input of said second speaker,

wherein the information signal of said second microphone is coupled to the input of said second amplifier to provide an amplified information signal output of said second amplifier, said amplifier information signal being coupled to said first speaker,

first adjusting means coupling a portion of the output of said second amplifier to said first amplifier to adjust the gain of said first amplifier as a function of the magnitude of said second amplifier information signal output,

second adjusting means coupling the output of said first amplifier to said second amplifier to adjust the gain of said sec-ond amplifier as a function of the magnitude of said first amplifier information signal output,

each of said adjusting means com-prising a rectifying circuit to provide a bias that serves to decrease amplifier gain, said rectifying circuit including a diode having a first terminal connected to the output of one of said amplifiers, a first resistor connected at its first end to the second terminal of said diode, a second resistor connected at its first end to the second end of said first resistor and at its second end to the input stage of the other of said amplifiers, and a capacitor connected at its first end to said second end of said first resistor and at its second end to ground,

first circuit means coupled to the output of said first microphone and to said first amplifier to provide a by-pass for a selected upper frequency range of said first microphone output, said by-pass being disabled by said first amplifier output, and

second circuit means coupled to the output of said sec-ond microphone and to said second amplifier to provide a by-pass for a selected upper frequency range of said second microphone, said by-pass being disabled by said second amplifier output.

'8. An intercommunication system comprising a first station having a first microphone, a first amplifier and a first speaker,

a second station having a second microphone, a second amplifier and a second speaker,

said first microphone being coupled to the input of said first amplifier and the output of said first amplifier being coupled to said second speaker, whereby speech at said first station may be amplified and transmitted to a listener at said second station,

said second microphone being coupled to the input of said second amplifier and the output of said second amplifier being coupled to said first speaker, whereby speech at said second station may be amplified and transmitted to a listener at said first station,

first rectifying circuit coupled to the output of said first amplifier and to an input stage of said second amplifier to rectify a portion of said first amplifier output and thereby provide a reverse bias which is applied to decrease the gain of said second amplifier, said rectifying circuit including a rectifier in series with an RC filter and time delay circuit,

second rectifying circuit coupled to the output of said second amplifier and to an input stage of said first amplifier to rectify a portion of said second amplifier output and thereby provide a reverse bias which is applied to decrease the gain of said second amplifier, said rectifying circuit including a rectifier in series with an RC filter and time delay circuit,

a shunting circuit, one each connected across the output of each of said microphones, said shunting circuit comprising a capacitor in series with a diode, said diode being connected to ground and one terminal of said capacitor being connected to the output of said microphone,

means for applying a bias to the ungrounded terminal of said diode, and

circuit means connected at One end to the output of said amplifier and at the other end to said ungrounded terminal of said diode to apply a voltage tending to cancel said bias, the magnitude of said canceling voltage being a function of the magnitude of the output of said amplifier, said circuit means having a relatively short time constant on the build up of said canceling voltage from said amplifier output, and havinga relatively long .time constant on the discharge of said canceling voltage.

References Cited by the Examiner UNITED STATES PATENTS ROBERT H. ROSE, Primary Examiner. 

1. IN AN INTERCOMMUNICATION SYSTEM FIRST STATION HAVING A MICROPHONE, AN AMPLIFIER COUPLED TO SAID MICROPHONE TO PROVIDE A STATION OUTPUT AND A SPEAKER ADAPTED TO RECEIVE A STATION INPUT FROM A SECOND STATION, SAID FIRST STATION ALSO HAVING CIRCUIT MEANS FOR COUPLING A PORTION OF THE INPUT OF SAID SPEAKER TO SAID AMPLIFIER TO REDUCE THE GAIN OF SAID AMPLIFIER AS A FUNCTION OF THE MAGNITUDE OF THE INFORMATION SIGNAL SUPPLIED TO SAID SPEAKER, THE IMPROVEMENT COMPRISING: BYPASS CIRCUIT MEANS COUPLED TO THE OUTPUT OF SAID MICROPHONE TO PROVIDE A BYPASS FOR A PRE-DETERMINED UPPER FREQUENCY RANGE OF THE OUTPUT OF SAID MICROPHONE, AND DISENABLING CIRCUIT MEANS COUPLED TO THE OUTPUT OF SAID AMPLIFIER TO PROVIDE A SIGNAL TO DISENABLE SAID BYPASS CIRCUIT MEANS WHEN THE OUTPUT OF SAID AMPLIFIER ATTAINS A PRE-DETERMINED VOLUME LEVEL. 